1. Field of the Invention
This invention relates to an apparatus for retaining a lens, and more particularly to a lens retaining apparatus which retains a lens by a lens holding frame (or a lens barrel) through an adhesive.
2. Prior Art
As a structure for retaining a lens by a holding frame, there are adopted a means for fixing the lens to the holding frame through a screw, a caulking, a tap ring or the like without using an adhesive, and a means for dynamically holding the lens by synergistic action of a proper notch portion formed on the outer peripheral part of the lens outside its effective aperture and an elastic protrusion formed on the lens holding frame at a position corresponding to the notch portion.
In the means for fixing the lens to the holding frame, however, optical strain is produced in the lens due to the fixing force to considerably degrade the optical characteristics. On the other hand, in the dynamically holding means, the centering adjustment can not be performed at a high accuracy, which is not adoptable for the lens holding part requiring a high accuracy centering.
In the lens system requiring a relatively high accuracy centering, therefore, there is generally adopted a structure that a lens 2 is fixedly held at a lens barrel (or a holding frame) 3 through an adhesive 1 as shown in FIGS. 7a and 7b.
In the embodiment of FIG. 7a, the adhesive 1 is filled in a space defined between the outer peripheral surface of the lens 2 and the inner peripheral surface of the lens barrel 3 along its whole circumference. In the embodiment of FIG. 7b, the adhesive 1 is dottedly filled in a space 4 defined between the outer peripheral surface of the lens 2 and the inner peripheral surface of the lens barrel 3 at plural equi-distant or irregular positions.
In the above embodiments, an ultraviolet-curing type adhensive is used as the adhesive 1, so that the lens can be fixedly held at a centered state with a relatively high accuracy. However, there are the following problems.
Namely, the holding force (bonding strength) of the lens 2 is a product of bonding strength of the adhesive 1 by adhession area (S=.pi.D.times.length of receiving portion, D: radius of lens). In this connection, the adhesion strength in FIG. 7a becomes excessively large due to the whole circumferential adhesion, so that the optical strain occurs in the lens 2 and consequently the optical characteristics after the fixing of the lens are considerably degraded as compared with the optical characteristics of the lens itself. In the embodiment of FIG. 7b, the adhesion strength may be set to a predetermined value. However, the dottedly applied adhesive 1 nonuniformly penetrates into the annular space 4 between the lens 2 and the lens barrel 3, so that there is caused the scattering in the adhesion strength among the dottedly applied portions of the adhesive 1, resulting in the increase of the optical strain.